1. Field of the Invention
The present invention relates to a detector for controlling a voltage pump. More particularly, the present invention relates to a high voltage detector for a voltage pump used in a circuit with low voltage process transistors, such as a circuit using 2.5 volt process transistors.
2. Description of the Related Art
FIG. 1 shows a typical circuit for a high voltage detector used to control a power supply voltage pump. The voltage pump is used to provide a voltage greater than provided by a power supply pin to a chip.
An example of an integrated circuit which might use a high voltage detector to control a voltage pump is described in U.S. patent application Ser. No. 08/912,763. The circuit described in application Ser. No. 08/912,763 includes 2.5 volt process transistors and has a power supply pin voltage ranging from 3.0-3.6 volts. For 2.5 volt process transistors, typically a maximum gate to source, or gate to drain voltage is 2.7 volts. The circuit in Applicant Ser. No. 08/912,763 also requires a voltage 1 vt greater than the pin supply voltage. Note that 1 vt is the threshold of a CMOS transistor made using the 2.5 volt semiconductor process.
The high voltage detector of FIG. 1 functions to monitor the difference between a pin voltage V.sub.1 and a voltage V.sub.2 on a reference line which is to be 1 vt above V.sub.1. The circuit of FIG. 1 produces a signal (PUMPON) to turn on a charge pump when V.sub.2 on the reference line drops from 1 vt above V.sub.1.
The circuit of FIG. 1 includes an operational amplifier 100 with a +(noninverting) terminal connected through a resistor 100 to the V.sub.1 supply pin. A resistor 104 connects the +opamp terminal to V.sub.SS or ground. The -(inverting) terminal of the opamp 100 is connected through a resistor 106 and transistor 108 having its drain and gate tied together to the V.sub.2 reference line. The transistor 108 provides a 1 vt drop from V.sub.2 to node n1 to create a voltage at node n1 which will be equal to V.sub.1 when the V.sub.2 reference line is properly charged up. Another resistor 110 connects the -opamp input to V.sub.SS or ground. The resistors 100, 104, 106 and 110 have an equal value R
In operation, when the value on the V.sub.2 reference line less 1 vt falls below V.sub.1, PUMPON at the output of opamp 100 will go high to cause the voltage pump to turn on and drive the V.sub.2 reference line. When the voltage pump adequately charges the V.sub.2 line to 1 vt above V.sub.1, the PUMPON will go low to turn off the voltage pump.